The present invention relates to a pneumatic dosimeter for the dosage of pulverulent materials, for instance for supplying aluminum fluoride and oxide to an aluminum electrolytic cell. The dosimeter comprises a vessel or container with an upper chamber wherein is provided a fluidizing canvas or air permeable dividing plate with an underlying chamber which is connected to an air filter via an air supply pipe. The vessel is provided with an inlet for the supply of pulverulent material from a hopper or supply tank to the upper chamber, and an outlet wherethrough the chamber is being emptied.
Devices which provide for the transport and dosage of pulverulent materials by utilizing generally known fluidizing principals are known in the art. Such devices necessarily utilize materials that are fluidizable; that is, they utilize pulverulent materials which have a grannular geometry and mutual cohesiveness that allow the individual granules of the material to be slowly separated and suspended when air or other gas is blown into the material. Suspension of the granules in the air or other gas allows the material to behave like a homogeneous liquid. Materials having such properties and characteristics include aluminum oxide which is used in fused salt electrolytic cells, cement, gypsum, calcium flouride, carbon powder, sodium sulphate, phosphates, polyphosphates, pryophosphtes, metal powder, powered plastic material, and food products such as flour, powdered milk, sugar and the like.
Norwegian patent application No. 855,219, which corresponds to U.S. Pat. No. 4,692,086, reveals an apparatus for providing doses of pulverulent materials and consists of a housing with a fluidizing device. The upper part of the housing is at one end connected to a vessel via a column and on the other end provided with an equilibrium and degassing column. The fluidizing device is provided at the bottom of a fluidizing wall with an underlying chamber which is connected to an air source via an air pipe. The dosage of the powder in the housing is accomplished by creating a state of equilibrium in each of the columns at the ends of the housing. The state at which equilibrium is reached is dependent on the air pressure and the powder consistency. Thus, only small differences in air pressure and/or differences in powder consistency will result in significant changes in the amount of material being dosed. The aparatus is also complicated and is physically large in height. The large height represents a major disadvantage, for instance, when being used in connection with electrolytic cells of the Soederberg type which require the use of dosimeters which are low in height.
Norwegian patent No. 154401 discloses a mechanical dosimeter for pulverulent material. The dosimeter consists of a vessel with walls converging towards an outlet opening in the bottom of the vessel. The powder is discharged through the hole in the bottom via a mechanical device which consists of a cylinder/piston arrangement. Though the mechanical dosimeter gives relatively exact doses, it is encumbered with several disadvantages. For one thing, the mechanical mechanisms become worn and thus require extensive maintenance. Further, the pulverulent material, depending upon its consistency, may clog the outlet opening of the hopper or vessel. This dosimeter is also expensive to produce due to the fact that it consists of several different parts, and that it is large in height.